Semiconductor device having a fin transistor and method for fabricating the same

ABSTRACT

A fin transistor includes fin active region, an isolation layer covering both sidewalls of a lower portion of the fin active region, a gate insulation layer disposed over a surface of the fin active region, and a gate electrode disposed over the gate insulation layer and the isolation layer, and having a work function ranging from approximately 4.4 eV to approximately 4.8 eV.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority of Korean patent applicationnumber 10-2007-0026073, filed on Mar. 16, 2007, which is incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

The present invention generally relates to a fabricating technology of asemiconductor device, and more particularly, to a semiconductor devicehaving a fin transistor and a method for fabricating the same.

Presently, since semiconductor devices have become highly integrated,typical 2-dimensional transistors encounter limitations. In that,2-dimensional transistors are not available for high-speed devicesbecause they cannot meet a demand for high current drivability.

To overcome such limitations, a variety of transistors having a3-dimensional structure are being proposed, of which one is a fin fieldeffect transistor (fin-FET, hereinafter referred to as a fintransistor). The fin transistor uses three sides as a channel to improvecurrent drivability. On the contrary, the fin transistor is difficult tosecure a threshold voltage greater than a certain level due to itsthree-side channels. Therefore, it is difficult to apply the fintransistor to a cell transistor of a memory device such as a dynamicrandom access memory (DRAM) because off-leakage characteristics aredeteriorated unless securing a threshold voltage greater than apredetermined level in a memory device such as the DRAM.

Accordingly, a technique, which is capable of increasing a thresholdvoltage of a fin transistor, is required for employing the fintransistor as a cell transistor of the memory device. One of Thetechniques that has been suggested is a polysilicon layer, doped withp-type impurities such as boron (B), used as a gate electrode of the fintransistor instead of a typical polysilicon layer doped with n-typeimpurities such as phosphorous (P). Hereinafter, for convenience of thedescription, the polysilicon layer doped with p-type impurities will bereferred to as a P+ polysilicon layer and the polysilicon layer dopedwith n-type impurities will be referred to as a N+ polysilicon layer.Theoretically, since the P+ polysilicon layer has a work functiongreater than N+ polysilicon layer by approximately 1.1 eV, it ispossible to increase the threshold voltage of the fin transistor by acertain voltage level, e.g., approximately 0.8 V to approximately 1.0 V,by substituting the P+ polysilicon gate electrode for the N+ polysilicongate electrode. A typical semiconductor device having such a fintransistor is illustrated in FIG. 1.

FIG. 1 illustrates a cross-sectional view of a semiconductor devicehaving a typical fin transistor. Here, the semiconductor device,particularly a memory device, includes a cell region A and a peripheralregion B. The cell region is configured with an NMOS transistor. Theperipheral region B is divided into an NMOS peripheral region B1 and aPMOS peripheral region B2. A fin transistor having a typical structureis formed in the cell region A of the memory device, whereas a generalplanar transistor is formed in the peripheral region B.

As illustrated in FIG. 1, an isolation layer 12 is formed in a substrate11 to define a first active region 11A in the cell region A, a secondactive region 11B in an NMOS peripheral region B1 of a peripheralcircuit, and a third active region 11C in a PMOS peripheral region B2 ofthe peripheral circuit. The first, second and third active regions 11A,11B and 11C are separated from one another by the isolation layer 12. Aportion of the isolation layer 12 in the cell region A where a gateelectrode will pass, is removed through masking and etching processes toform a gap G, thus exposing a top surface and portions of sidewalls ofthe first active region 11A. The first active region 11A verticallyprotrudes from the substrate 11 in virtue of the gap G. This protrudingfirst active region serves as a fin active region in the fin transistor.

Second and third gate insulation patterns 13B and 13C and second andthird gate conductive patterns 14B and 14C are sequentially formed overthe second and third active regions 11B and 11C, respectively. Thesecond gate conductive pattern 14B of the NMOS peripheral region B1 isformed of N+ polysilicon having a low work function, and the gateelectrode 14C of the PMOS peripheral region B2 is formed of P+polysilicon having a high work function.

A first gate insulation pattern 13A is formed on a surface of theexposed first active region 11A. A first gate conductive pattern 14A isformed on the first gate insulation pattern 13A and the isolation layer12 in the cell region A such that it overlaps the gap G while crossingthe first active region 11A. The first gate conductive pattern 14A inthe cell region A is formed of P+ polysilicon, thus increasing thethreshold voltage of the fin transistor.

However, the typical semiconductor device has several limitations below.In general, the P+ polysilicon has the work function greater than 4.8 eVand the N+ polysilicon has the work function smaller than 4.4 eV. Forinstance, it is assumed that there are two cases, i.e., one case where aP+ polysilicon gate having a work function of approximately 5.2 eV isformed on a gate oxide layer and an n-type junction, and the other casewhere an N+ polysilicon gate having a work function of approximately 4.2eV is formed on a gate oxide layer and an n-type junction. An energyband diagram of each case is shown in FIG. 2. From these energy banddiagrams, it can be observed that a band bending phenomenon becomessevere at an interface between the gate oxide layer and an n-typejunction by a degree corresponding to a work function difference(φ_(P)-φ_(N)) between the P+ polysilicon and the N+ polysilicon in thecase of using a P+ polysilicon gate electrode. Further, gate induceddrain leakage (GIDL) characteristics become poorer in the case of usingthe P+ polysilicon gate electrode than the N+ polysilicon gateelectrode, thus deteriorating data retention characteristics of a memorydevice. Therefore, to apply the fin transistor with improved currentdrivability as the cell transistor of the memory device, it is necessaryto develop a technology capable of minimizing a band bending phenomenonwhile increasing a threshold voltage to a certain level or higher.

SUMMARY OF THE INVENTION

The present invention contemplates a semiconductor device and a methodfor fabricating the same, which can improve device characteristics suchas gate induced drain leakage (GIDL), data retention and currentdrivability by securing a threshold voltage to a certain level or higherand minimizing a band bending phenomenon at an interface between a gateoxide layer and an n-type junction as well, using a material having awork function smaller than P+ polysilicon but greater than N+polysilicon for a gate electrode of a fin transistor.

In accordance with a first aspect of the present invention, there isprovided a fin transistor including fin active region, an isolationlayer covering both sidewalls of a lower portion of the fin activeregion, a gate insulation layer disposed over a surface of the finactive region, and a gate electrode disposed over the gate insulationlayer and the isolation layer, and having a work function ranging fromapproximately 4.4 eV to approximately 4.8 eV.

In accordance with a second aspect of the present invention, there isprovided a semiconductor device including a substrate having first,second and third regions with respective active regions which areseparated from one another by an isolation layer. The active region ofthe first region being provided as a fin active region. A gateinsulation layer formed over the active regions of the first throughthird regions, and first, second and third gate electrodes disposed overthe substrate of the first through third regions. A a fin transistor isprovided in the first region, and work functions of the first throughthird gate electrodes are different from one another, the first gateelectrode having the work function between those of the second and thirdgate electrodes.

In accordance with a third aspect of the present invention, there isprovided a method for fabricating a fin transistor, the method includesforming an isolation layer in a substrate to define an active region,selectively etching a portion of the isolation layer where a gateelectrode passes, to form a fin active region, forming a gate insulationlayer over the surface of the fin active region, and forming a gateelectrode over the gate insulation layer and the isolation layer, thegate electrode having a work function ranging from approximately 4.4 eVto approximately 4.8 eV.

In accordance with a fourth aspect of the present invention, there isprovided a method for fabricating a semiconductor device, the methodincludes forming an isolation layer in a substrate having first, secondand third regions, to define active regions in the first through thirdregions, respectively, selectively etching a portion of the isolationlayer in the first region where a gate electrode passes, to form a finactive region, forming a gate insulation layer over the fin activeregion of the first region and the active regions of the second andthird regions, thereby forming a first resultant structure, and formingfirst, second and third gate electrodes over the substrate of the firstthrough third regions, wherein work functions of the first through thirdgate electrodes are different from one another, and the first gateelectrode has the work function between those of the second and thirdgate electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of a semiconductor devicehaving a typical fin transistor.

FIG. 2 illustrates energy band diagrams of the cases where N+polysilicon and P+ polysilicon are used as a gate electrode material ofthe typical fin transistor, respectively.

FIG. 3 illustrates a cross-sectional view of a semiconductor devicehaving a fin transistor in accordance with an embodiment of the presentinvention.

FIGS. 4A to 4I illustrate a method for fabricating the semiconductordevice of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 illustrates a cross-sectional view of a semiconductor devicehaving a fin transistor in accordance with an embodiment of the presentinvention. In particular, a memory device is exemplarily illustratedhereinafter. The memory device of FIG. 3 includes a cell region A and aperipheral region B. The cell region A is configured with an NMOStransistor. The peripheral region B is divided into an NMOS peripheralregion B1 and a PMOS peripheral region B2. It is preferable that a fintransistor is formed in the cell region A of the memory device. Althoughtransistors having a variety of structures may be formed in theperipheral region B, a planar transistor is formed in the peripheralregion in the present invention, for example.

Referring to FIG. 3, an isolation layer 32 is provided in a substrate 31to define a first active region 31A in the cell region A, a secondactive region 31B in the NMOS peripheral region B1, and a third activeregion 31C in the PMOS peripheral region B2. The first, second and thirdactive regions 31A, 31B and 31C are separated from one another by theisolation layer 32. A portion of the isolation layer 32 in the cellregion A where a gate electrode will pass is removed through masking andetching processes to form a gap G, thus exposing a top surface andportions of sidewalls of the first active region 31A. The first activeregion 31A vertically protrudes from the substrate 31 in virtue of thegap G. This protruding first active region 31A serves as a fin activeregion in the fin transistor.

A first gate insulation pattern 33A is disposed over the exposed firstactive region 31A. A first gate conductive pattern 34 is disposed overboth the first gate insulation pattern 33A and the isolation layer 32 inthe cell region A such that it overlaps the recess G while crossing thefirst active region 31A. The first gate conductive pattern 34 of thecell region A serves as a first gate electrode. The first gateconductive pattern 34 is formed of a material having a work functionsmaller than P+ polysilicon but greater than N+ polysilicon, that is, inthe range from approximately 4.4 eV to approximately 4.8 eV.Resultantly, it is possible to secure a threshold voltage to a certainlevel or higher and minimize a band bending as well, even if a fintransistor is employed in the cell region A. It is preferable that thefirst gate conductive layer 34A is formed of SiGe containing the contentof Ge ranging from approximately 40% to approximately 70% in a thicknessrange from approximately 800 Å to approximately 1,200 Å.

A second gate insulation pattern 33B and a second gate conductivepattern 35N are sequentially formed over the second active region 31B inthe NMOS peripheral region B1. Likewise, a third gate insulation pattern33C and a third gate conductive pattern 35P are sequentially formed overthe third active region 31C in the PMOS peripheral region B2. The secondgate conductive pattern 35N includes an N+ polysilicon layer that has awork function smaller than approximately 4.4 eV and is doped with n-typeimpurities such as phosphorous (P) or arsenic (As). The third gateconductive pattern 35P includes a P+ polysilicon layer that has a workfunction greater than approximately 4.8 eV and is doped with p-typeimpurities such as boron (B).

Herein, the first, second and third gate conductive layers 34, 35N and35P may further include a low-resistance conductive layer such astungsten (W) and tungsten silicide (WSi_(x)) thereon. In thesemiconductor memory device of FIG. 3, since the gate electrode of thefin transistor formed in the cell region is formed of a material havinga work function between those of N+ polysilicon and P+ polysilicon,i.e., in the range of approximately 4.4 eV to approximately 4.8 eV, itis possible to improve device characteristics.

FIGS. 4A to 4I illustrate a method for fabricating the semiconductordevice of FIG. 3.

Referring to FIG. 4A, an isolation layer 42 is formed in a substrate 41,thus defining a first active region 41A in a cell region A, a secondactive region 41B in an NMOS peripheral region B1 and a third activeregion 41C in a PMOS peripheral region. The isolation layer 42 may beformed through a shallow trench isolation (STI) process.

To form a fin transistor in the cell region A, a portion of theisolation layer 42 in the cell region A where a gate electrode will passis selectively etched to form a gap G. Accordingly, a top surface andportions of sidewalls of the first active region 41A are exposed. Thefirst active region 41A after the selective etch serves as a fin activeregion of a fin transistor.

A first gate interlayer insulation pattern 43A is formed on the surfaceof the exposed first active region 41A, a second gate interlayerinsulation pattern 43B is formed over the second active region 41B, anda third gate interlayer insulation pattern 43C is formed over the thirdactive region 41C. Thereafter, a first gate conductive layer 44 isformed of a material having a work function ranging from approximately4.4 eV to approximately 4.8 eV over a resultant structure. Preferably,the first gate conductive layer 44A is formed of SiGe containing thecontent of Ge ranging from approximately 40% to approximately 70% in athickness range of approximately 800 Å to approximately 1,200 Å.

Referring to FIG. 4B, a first photoresist pattern 45 is formed over thefirst conductive layer 44 such that it covers the cell region A butexposes the peripheral region B. Thereafter, the first conductive layer44 is etched using the photoresist pattern 45 as an etch mask, thusleaving a first gate conducive pattern 44A only in the cell region A.

Referring to FIG. 4C, the photoresist pattern 45 and the first andsecond gate interlayer insulation patterns 43B and 43C are removed, anda first gate insulation pattern 46A is formed over the first gateconductive pattern 44A in the cell region A, and second and third gateinsulation patterns 46B and 46C are respectively formed over the secondand third active regions 41B and 41C in the peripheral region B.

Referring to FIG. 4D, an undoped polysilicon layer 47 is formed over aresultant structure. The undoped polysilicon layer 47 is formed to athickness ranging from approximately 1,000 Å to approximately 1,500 Åalong a surface profile of the resultant structure in the cell region Aand the peripheral region B.

Referring to FIG. 4E, a planarization is performed to expose the surfaceof the first gate conductive pattern 44A of the cell region A using, forexample, chemical mechanical polishing (CMP). As a result, a planarizedundoped polysilicon pattern 47A is formed in the peripheral region B,which are separated from the first gate conductive pattern 44A of thecell region A by a first gate insulation bar pattern 46D.

Referring to FIG. 4F, a second photoresist pattern 48 is formed over thefirst gate conductive pattern 44A and a portion of the planarizedundoped polysilicon pattern 47A such that it covers the cell region Aand the NMOS peripheral region B1 but exposes the PMOS peripheral regionB2. Afterwards, p-type impurities such as boron (B) are ion-implanted byusing the second photoresist pattern 48 as an ion implantation mask.Therefore, the planarized undoped polysilicon pattern 47A of the PMOSperipheral region B2 is converted into a P+ polysilicon layer doped withthe p-type impurities. Hereinafter, the P+ polysilicon layer will bereferred to as a third polysilicon region 47P for a third gateelectrode. The P+ polysilicon layer, the third polysilicon region 47P,has a work function greater than approximately 4.8 eV.

Referring to FIG. 4G, a third photoresist pattern 49 is formed over thefirst gate conductive pattern 44A and the third polysilicon region 47Psuch that it covers cell region A and the PMOS peripheral region B2 butexposes the NMOS peripheral region B1. Thereafter, n-type impuritiessuch as arsenic (As) are ion-implanted by using the third photoresistpattern 49 as an ion implantation mask. Therefore, the planarizedundoped polysilicon pattern 47A of the PMOS peripheral region B2 isconverted into an N+ polysilicon layer doped with the n-type impurities.Hereinafter, the N+ polysilicon layer will be referred to as a secondpolysilicon region 47N for a second gate electrode. The N+ polysiliconlayer, the second polysilicon region 47N, has a work function smallerthan approximately 4.4 eV.

Referring to FIG. 4H, a low-resistance layer 50 is formed of tungsten(W) or tungsten silicide (WSi_(x)) over the first gate conductivepattern 44A and the second and third polysilicon regions 47N and 47P.

Referring to FIG. 4I, the first gate conductive pattern 44A, the secondand third polysilicon regions 47N and 47P, and the tungsten (W) layer(or the tungsten silicide (WSi_(x)) layer) 50 are patterned to form gateelectrodes. That is, a first gate electrode is formed in the cell regionA, which is configured with a patterned first gate conductive pattern44A and a first low-resistance pattern 50A, i.e., tungsten (W) patternor tungsten silicide (WSi_(x)) pattern. Likewise, a second gateelectrode is formed in the NMOS peripheral region B1, which isconfigured with a second gate conductive pattern 47NA and a secondlow-resistance pattern 50B, i.e., tungsten (W) pattern or tungstensilicide (WSi_(x)) pattern, and a third gate electrode is formed in thePMOS peripheral region B2, which is configured with a third gateconductive pattern 47PA and a third low-resistance pattern 50C, i.e.,tungsten (W) pattern or tungsten silicide (WSi_(x)) pattern. The firstgate electrode crosses the cell active region 41A and overlaps the gapG.

Consequently, a fin transistor is formed in the cell region A and aplanar transistor is formed in the peripheral region B. Since the firstgate conductive pattern 44A of the fin transistor has a work functionbetween those of the second and third gate conductive patterns 47NA and47PA, it is possible to increase the threshold voltage of thesemiconductor device having the fin transistor and minimize a bandbending phenomenon as well.

Although the embodiments described herein illustrate the semiconductormemory device in which the fin transistor is formed in the cell regionand the planar transistor is formed in the peripheral region, thepresent invention is not limited to them. Hence, the present inventioncan be also applied to a variety of semiconductor integrated circuitsbesides the memory devices.

In a semiconductor device and a method for fabricating the same inaccordance with the present invention, it is possible to improve devicecharacteristics such as gate induced drain leakage (GIDL), dataretention and current drivability by securing a threshold voltage to apredetermined level or higher and minimizing a band bending phenomenonat an interface between a gate oxide layer and an n-type junction aswell, using a material having a work function smaller than P+polysilicon but greater than N+ polysilicon for a gate electrode of afin transistor.

While the present invention has been described with respect to thespecific embodiments, the above embodiments of the present invention areillustrative and not limitative. It will be apparent to those skilled inthe art that various changes and modifications may be made withoutdeparting from the spirit and scope of the invention as defined in thefollowing claims.

1. A fin transistor, comprising: a fin active region; an isolation layercovering both sidewalls of a lower portion of the fin active region; agate insulation layer disposed over a surface of the fin active region;and a gate electrode disposed over the gate insulation layer and theisolation layer, and having a work function ranging from approximately4.4 eV to approximately 4.8 eV.
 2. The fin transistor of claim 1,wherein the fin transistor includes an NMOS transistor.
 3. The fintransistor of claim 1, wherein the gate electrode includes a silicongermanium (SiGe) layer.
 4. The fin transistor of claim 3, wherein thecontent of Ge is in the range of approximately 40% to approximately 70%in the SiGe layer.
 5. The fin transistor of claim 3, wherein the SiGelayer has a thickness ranging from approximately 800 Å to approximately1,200 Å.
 6. The fin transistor of claim 3, wherein the gate electrodefurther comprises a low-resistance metal layer over the SiGe layer. 7.The fin transistor of claim 6, wherein the low-resistance metal layerincludes one of a tungsten (W) layer and a tungsten silicide (WSix). 8.A semiconductor device, comprising: a substrate including first, secondand third regions having respective active regions which are separatedfrom one another by an isolation layer, wherein an active region of thefirst region being provided as a fin active region; a gate insulationlayer formed over the active regions of the first through third regions;and first, second and third gate electrodes disposed over the substrateof the first through third regions respectively, wherein a fintransistor is provided in the first region, and work functions of thefirst through third gate electrodes are different from one another, thefirst gate electrode having the work function between those of thesecond and third gate electrodes.
 9. The semiconductor device of claim8, wherein the first region is a cell region, the second region is anNMOS peripheral region, and the third region is a PMOS region.
 10. Thesemiconductor device of claim 8, wherein the work function of the firstgate electrode is in the range of approximately 4.4 eV to approximately4.8 eV, the work function of the second gate electrode being smallerthan approximately 4.4 eV and the work function of the third gateelectrode being greater than approximately 4.8 eV.
 11. The semiconductordevice of claim 10, wherein the first gate electrode includes a silicongermanium (SiGe) layer.
 12. The semiconductor device of claim 11,wherein the content of Ge is in the range of approximately 40% toapproximately 70% in the SiGe layer.
 13. The semiconductor device ofclaim 11, wherein the SiGe layer has a thickness ranging fromapproximately 800 Å to approximately 1,200 Å.
 14. The semiconductordevice of claim 10, wherein the second gate electrode includes an N+polysilicon layer, and the third gate electrode includes a P+polysilicon layer.
 15. The semiconductor device of claim 10, whereineach of the first through third gate electrodes further comprises alow-resistance metal layer thereover. 16-30. (canceled)